1. Field of the Invention
The invention relates in general to methods and devices for applying a non-thermal plasma treatment to objects to change the object's surface characteristics, and, more particularly, embodiments of the present invention relate to treating solid surfaces in a substantially sealed dielectric barrier discharge plasma chamber system with exterior electrodes.
2. Description of the Prior Art
The use of non-thermal plasma to modify the surface properties of many solids for many different purposes is well known. Thermal plasma generators typically operate at temperatures where most metals melt or vaporize, so they are unsuitable for use with organic polymers, and the like. Non-thermal plasma treatment of the surfaces of objects changes the electrical charge, physical and/or chemical properties of the surface. Such properties include, for example, surface tension, biocompatibility, functionality, and the like. See, for example, Murokh U.S. Pat. No. 5,798,146. Depending on the composition of the atmosphere in the region where the non-thermal plasma forms, chemical modification of the surface may be accomplished. Deposits that are only a few molecular layers thick may be formed. At high power settings, the surface of the workpiece may be etched or damaged. Various systems for generating non-thermal plasma had been previously proposed.
It is well known that when two or more electrodes are spaced apart to form a gap between them, the application of oscillating high voltage across them will, under the right conditions, cause a non-thermal plasma or glow discharge to form. The plasma forms when the breakdown voltage is exceeded. The breakdown voltage of a plasma (glow discharge) system is dependent upon the width of the gap between the electrodes, the pressure and characteristics of the atmosphere in that gap as well as the frequency of the applied voltage. Generally, the minimum voltage required to sustain the glow discharge decreases with increasingly higher frequencies. At constant atmospheric conditions, the breakdown voltage decreases as the width of the gap decreases. The breakdown voltage also decreases as the pressure of the atmosphere decreases. The breakdown voltage also depends on the nature of the gas between the electrodes. For example, at atmospheric pressure, the breakdown voltage for neon or argon filled gaps is much lower than for air filled gaps. For a given system of electrodes, gap and atmosphere, the required power density (Watts per square centimeter) is largely dictated by the purpose for which the plasma is applied to the surface. Where the surface is to be etched or otherwise physically changed, the power density is much higher than where only the electrical charge of the surface is to be altered.
Numerous non-thermal plasma generator configurations had been previously proposed. One such prior system comprises a vacuum chamber in which bare electrodes are connected to an oscillating high voltage source. See, for example, FIG. 1 where such a system is indicated generally at 10. System 10 includes a vacuum chamber 19, which is evacuated to a pressure in the millitorr range by vacuum pump 22 through vacuum line 24. A source of oscillating high voltage electrical energy 26 is connected by electrical leads 28 and 30 to a pair of bare electrodes located within chamber 19. Plasma forms within the chamber 19 between the bare electrodes. Such systems are expensive to make. The electrodes must be built into the chamber and the electrical leads carefully sealed to hold the vacuum. Expensive seals are required. Precision in construction is required, and the precision must be maintained in operation, which requires very skilled operators. Expensive two stage vacuum pumps are typically required to draw the pressure down to acceptable millitorr operating levels. Operating expenses are high because of the pumps, the maintenance, and the long cycle times, which tend to be in the order of several minutes. Such systems are typically too expensive to build to fit one particular part. They are designed as general-purpose plasma treatment generators. For this reason there is a large gap between the electrodes, which dictates that the pressure must be reduced to the millitorr range. A particular workpiece may not fit well in the chamber, so it does not receive a uniform surface treatment, or it may leave a lot of empty space in the chamber. This empty space must be evacuated at considerable operating expense. Such systems are designed to operate in a batch mode.
Dielectric barrier discharge plasma generating systems are well known. Dielectric barriers serve to promote the formation of a more uniform plasma. Dielectric barrier discharge systems are characterized by the presence of a dielectric barrier associated with at least one of the electrodes. Often the dielectric barrier is provided as a coating on one or more of the electrodes. Dielectric barriers in dielectric barrier discharge systems are typically less than approximately 0.1 inches thick, and may be as thin as approximately 10 microns or less. For purposes of illustration, the dielectric barriers illustrated in the Figs. herein are usually shown as being thicker than they actually are.
A prior dielectric barrier discharge plasma generator system is depicted diagrammatically in FIG. 2. The depicted dielectric barrier discharge system (DBD) indicated generally at 12 includes a pair of electrodes 38 and 40, connected via electrical leads 44 and 46 to a source 42 of oscillating high voltage electrical energy. Dielectric barriers 34 and 36 are interposed between at least one of electrodes 38 and 40, respectively, and the region 48 where plasma forms. The system operates at atmospheric pressure without a vacuum chamber. It is very hard to maintain a stable glow discharge between the electrodes in air, because of significant plasma instabilities. At atmospheric pressure in air, plasma has a streamer like pattern with highly non-uniform volume energy distribution. In order to sustain a uniform volume discharge at atmospheric pressure for gaps between the electrodes of about 1 centimeter (cm) and more, special air/helium mixtures, or the like, are generally required. Argon or helium may be injected into the gap where the plasma forms so as to control the atmosphere within the gap. It is practically impossible to form a uniform plasma in air at atmospheric pressure for gaps of more than approximately 5 cm. Continuous webs are often treated using a system of the type schematically depicted in FIG. 2.
Yet another previously proposed form of plasma generator is diagrammatically depicted in FIG. 3. The system depicted generally at 14 is a hybrid in that it includes a vacuum chamber 19 like the embodiment depicted in FIG. 1, and a dielectric barrier discharge configuration as depicted in FIG. 2. At least one of electrodes 38 and 40 is provided with a dielectric facing such as shown at 34 and 36, respectively. The electrodes are connected to a source 50 of oscillating high voltage by way of electrical leads 52 and 54, respectively. The chamber 19 is evacuated to a very low pressure by vacuum pump 22 through vacuum line 24. The application of oscillating high voltage electrical energy to electrodes 38 and 40 causes plasma to form in the region 56 between the electrodes.
Vacuum chambers with electrodes within the chamber in dielectric barrier discharge systems are well known, but they are expensive to buy and operate, because they typically must operate in the millitorr pressure range and at high voltages. This is because of large gaps (3 inches or more) between the electrodes that are required to accommodate a variety of different workpieces. Also, the construction of in-chamber electrode systems is expensive. The chambers with internal electrodes are generally too expensive to custom build to fit a single part, so they are made big enough to accommodate a variety of different parts or a plurality of parts of the same configuration. They require expensive two stage pumps to bring the pressure down into the millitorr range. Cycle times are in the nature of minutes, and they are usable only in a batch processing mode.
Some in-chamber dielectric barrier discharge systems have been proposed for single purpose operations. See, for example, De Vries et al. US 2006/0147648, Pub. Jul. 6, 2006. De Vries proposes the use of a dielectric barrier discharge system wherein a special gas atmosphere (for example, air, argon, nitrogen, oxygen, carbon dioxide, ammonium, common precursors, etc.) is confined within a chamber around a thermoplastic film. The surface of the thermoplastic film is treated with plasma as it is transported past a set of electrodes. The electrodes on one side are coated with a dielectric, and the film is supported by the bare electrode on the other side. The chamber is at atmospheric pressure and vents directly to the surrounding atmosphere, although it is mentioned without amplification that the process may be operated below atmospheric pressure such as between 100 mbar and 1 bar (approximately between 75 and 750 Torr) pressure. It is not clear how the system could be modified to operate at reduced pressures. The gap between the electrodes and the workpiece film is described to be in the range of 0.1-5 millimeters, which is indicative of operation at or near atmospheric pressure.
Dielectric barrier discharge systems had been proposed for the treatment of gases flowing axially within a stationary dielectric tube. See, for example, Shiloh et al. U.S. Pat. No. 6,245,299. Shiloh discloses a series of electrode sets, each set being axially spaced from its neighbors, all of which are arrayed along a stationary dielectric tube. The stationary dielectric tube is located between the electrodes of each set so that a non-thermal plasma is caused to form inside of the tube at the sites where the sets of electrodes are located. A polluted gas stream flows through the tube and is serially treated by plasma as it passes each set of electrodes. The composition of the polluted gas stream is changed as it passes through the plasma. A similar stationary dielectric barrier discharge system for treating a flowing gas stream is described in Lazarovich et al. U.S. Pat. No. 6,685,803.
Hammerstrom et al. U.S. Pat. No. 6,455,014 discloses a non-thermal plasma generating blanket that may be draped over a contaminated surface. The blanket is in the form of a dielectric barrier discharge system.
Jacob U.S. Pat. No. 6,342,187 discloses the use of a number of dry sterilization procedures, one of which is described as plasma glow. The devices that are said to be sterilized include small elongated cylindrical medical devices such as fiber optics devices. The teaching appears to be that such elongated devices should be placed in some outside container, and the sterilization procedure applied in that container in a batch procedure.
Nakamura et al. U.S. Pat. No. 6,489,585 describes the dielectric barrier discharge plasma treatment of the surface of a moving glass substrate for purposes of cleaning the surface. The plasma is generated in a treatment chamber.
A typical prior plasma generating system might require, for example, a source of electrical energy that provides 1,000 Volts (1 KV) and a current (for example, 1.0 Amp) to establish a glow discharge (plasma) in a vacuum with a gap between the electrodes of about 4 inches. Dielectric barrier discharge at atmospheric pressure might require 10 KV and 0.1 Amp to create plasma in a gap of less than one-quarter inch. Breakdown voltages vary with the pressure. For example, typical breakdown voltages for about a 1 centimeter (cm) gap at different pressures are shown in the following table.
TABLE 1Breakdown Voltages (KV) for 1 cm gapPressure (Torr)AirArgon760303100611010.310.30.2
The effect of the composition of the atmosphere on the breakdown voltage decreases as the pressure is reduced.
These and other difficulties of the prior art have been overcome according to the present invention.